Ostreogrycin A (virginiamycin M1, streptogramin A) is the major component of the virginiamycin complex. In the 1950s this complex was independently discovered so many times that the literature became highly confusing. Ostreogrycin A is a macrocyclic lactone antibiotic that acts synergistically with the structurally unrelated cyclic depsipeptides, virginiamycin B (ostreogrycin B, streptogramin B) and virginiamycin S, to inhibit peptide elongation. This is achieved by blocking formation of a peptide bond between the growing peptide chain (peptidyl-tRNA) linked to the 50S ribosome and aminoacyl-tRNA. Ostreogrycin A is highly active against Gram positive bacteria, particularly MRSA.